Muslims are taking root in America. Though small in number, they're growing fast and setting up enclaves in some of the largest cities. A study released today shows they're better educated and almost as well paid as the non-Hispanic white population. But if American Muslims are poised to join the mainstream economically, politics and religion are roiling the confluence.

The aftermath of Sept. 11 and tension with Iraq has exposed a dual response to Muslims living here. There's both suspicion and outreach  discrimination and efforts to bridge the religious divide.

"Islam has become a part of public discourse and people are making up their minds," says Mohamed Nimer, author of a new book, "The North American Muslim Resource Guide." "America is confronting ... what we might call a precursor to being a truly pluralistic society."

While no one knows how many Muslims live in the US  estimates span 1.2 million to 10 million  their numbers are growing fast. Between 1990 and 2000, the Muslim-origin population grew 40 percent, according to the new study by the Lewis Mumford Center for Comparative Urban and Regional Research at New York's University at Albany. And they're more integrated than larger minorities.

"They're rather highly concentrated in certain metropolitan areas," says John Logan, Mumford Center director. "But in those areas they tend to live in neighborhoods where they're a distinct minority."

Four metropolitan regions  Los Angeles, New York, Detroit, and Washington  boast more than 100,000 American Muslims. A decade earlier, only Los Angeles could make that claim. Detroit's population includes more Muslims than virtually any city. But much smaller Jersey City, N.J., which saw a doubling of its Muslim population in the 1990s, now boasts the identical concentration: 2.8 percent.

Tricky to count  and difficult to see

Even with recent high-profile incidents  such as the arrests of suspected Islamic terrorists in Lackawanna, N.Y., and the detention of three hapless Arab medical students in Florida  the Muslim population remains relatively invisible. One reason is that it's hard to count. Although the nation contains some 500,000 indigenous Muslims  mostly black Muslims  most of the population's growth comes from immigration. And since the US census doesn't characterize religion, researchers have to make rough estimates based on place of birth and ancestry. That's tricky work.

Officially, for example, today's Mumford Center report doesn't count Muslims at all, but "Americans with roots in historically Islamic countries." So it includes immigrants from overwhelmingly Muslim Pakistan but leaves out India, with its huge Muslim minority. It includes Lebanon even though the first wave of Lebanese who came here a century ago were mostly Christian. Because of limitations in ancestry data, the study can count only US residents born in Indonesia (the world's largest Islamic country), and not their children.

To get around such limitations, another researcher has surveyed the roughly 1,200 American mosques and emerged with an estimate of 6 million Muslims. Dr. Nimer, a Palestinian by birth, argues there are no fewer than 2.4 million. The American Jewish Committee says there can be no more than 2.8 million.

Socioeconomic factors also make Muslims largely invisible here. They typically live in neighborhoods that are only 4.5 percent Muslim, according to the Mumford study, although that's changing as their numbers grow and they begin to create enclaves. Still, American Muslims are far less geographically segregated than Hispanics or African Americans. One possible factor: higher socioeconomic status. On average, they live in households earning $50,000 annually  more than any significant minority except Asian, and only $2,000 less than non-Hispanic white households, according to Mumford data. American Muslims have lower poverty rates than either blacks or Hispanics, and more years of education than any of these groups, including whites and Asians.

That may change, warns Nimer, because recent Muslim refugees from places like Somalia have acquired fewer skills.

More important, last year's terrorist attacks complicated acceptance of Muslims.

Sept. 11 "gave permission to the public and some police to exercise their prejudiced behavior against us," says Sofian Zakkout, director of the American Muslim Association of North America in Hialeah, Fla. Despite his standing in the community (he's also on the state Advisory Committee to the US Commission on Civil Rights), Mr. Zakkout has experienced the challenge firsthand. Last year, he was stopped by police after visiting a friend in the hospital. More recently, three landlords refused to rent office space to his organization.

"There is general fear in the community," adds Muzammil Siddiqi, director of the Islamic Society of Orange County in California. "The average American, I think, would like to see better relations. But there is a small group that wants to create suspicions."

One area of worry: evangelical Christians. The former president of the Southern Baptist Convention called Muhammad a "demon-possessed pedophile." Franklin Graham, who heads the ministry of his father, Billy Graham, has called Islam evil.

But the number of local interfaith meetings has risen, Dr. Siddiqi says  part of growing interchange nationwide. "We don't talk about 'Americans' " now, adds Nimer. "We talk about Catholics, African-Americans, and mainline Protestants.... Muslims are growing very sensitive to those differences. And they see themselves as being one part of this mosaic."